Reduced engine operation times in hybrid vehicles enable fuel economy and fuel emissions benefits. However, having the internal combustion engine off when the vehicle is driven in an electric mode and then performing a smooth engine restart can delay the delivery of torque from the engine. The delay can be exacerbated in engines operating on a Miller cycle where a boosting device (such as a turbocharger or a supercharger) has to spool up before delivering high pressure air to the air intake system.
One example approach for reducing the delay in turbocharger spool-up (also referred to as turbo lag) is shown by Berger et al. in U.S. Pat. No. 7,314,043. Therein, compressed air is delivered from a dedicated source to an engine intake manifold during selected conditions to provide an increase in cylinder charge when a turbocharger has not yet attained sufficient speed to provide the desired boost pressure. In this way, flow amplification is implemented until the turbocharger comes up to speed.
However, the inventors have identified potential issues with such an approach. As an example, hybrid vehicles may typically use under-sized engine systems. Since the turbocharger in such engine systems are spooled up from exhaust pressure, there may be considerable delay in achieving maximum torque. While adding compressed air to the intake, as per the approach of Berger, may reduce the time required for the turbocharger to spool-up, engine restart may still be delayed due to the added delay of starting and cranking the engine after the turbocharger has spooled up. For example, if the compressed air is added too early or too late, it may be wasted. In hybrid vehicles where the engine torque is required to meet an elevated driver demand, the combined delay in engine starting can lead to a substantial delay in maximum torque output and poor vehicle torque response. In addition, since motor torque is used to meet the torque demand before the engine is started, the system battery may discharge faster. Overall, undersized engines using turbo-boosted power may be not able to deliver impressive acceleration for up to several seconds.
In one example, some of the above issues may be addressed by a method for a hybrid vehicle comprising: during vehicle operation using motor torque to drive vehicle wheels, in response to a tip-in to wide-open-pedal, restarting the engine, and during engine cranking of the restarting, discharging compressed air from an accumulator to the engine while operating with advanced intake valve timing. In this way, camshaft timing adjustments and compressed air injection can be coordinated to reduce both the delay in turbocharger spool up and engine restart.
As an example, a hybrid vehicle system may include an engine and a motor. The engine may operate on a Miller engine cycle. During conditions when the operator torque demand is lower, the engine may be switched off and the torque demand may be met by the system motor. A high pressure air accumulator may be coupled to the engine intake manifold via a valve. The high pressure air accumulator may have been filled during previous engine operating conditions by opening the valve slightly so that a portion of the compressed aircharge being delivered to the engine can be stored in the accumulator. Once filled, the valve may be closed. In response to an elevated operator torque demand (such as an operator pedal tip-in to a wide open pedal position where maximum torque is requested), the engine may be restarted to meet the maximum torque demand. Specifically, the accumulator valve may be opened (e.g., fully opened) and compressed air may be discharged from the pressure accumulator into the engine intake manifold while the engine is cranking and while an engine transmission is in gear (e.g., a forward gear). A timing of opening of the valve may be adjusted based on a first engine combustion event from rest such that the compressed air is discharged into the first firing cylinder during the initial fueling of the first firing cylinder (or into a first number of firing cylinders during the initial fueling of the engine). When discharged in coordination with the fueling of the first firing cylinder, the added compressed air enables a higher engine torque output to be achieved while also expediting turbine spool-up. While opening the accumulator valve, an intake camshaft timing may also be advanced (e.g., fully advanced) to a position that allows maximum airflow and thereby maximum power output. By adjusting the intake cam timing, substantially all the injected compressed air can be used for improving engine restart response, and none of the injected air is wasted. As such, during other operator pedal tip-ins, where the pedal is not displaced to the wide-open position, the accumulator valve may be maintained closed and the intake camshaft timing may follow a nominal schedule. For example, while the engine is cranked, the intake camshaft may be retarded to provide a smooth engine restart.
In this way, by advancing an intake camshaft timing during engine cranking while delivering compressed air from a pressure accumulator to an engine intake, a delay in the onset of maximal engine torque output can be reduced (e.g., by several seconds, such as ˜3 seconds). The timed addition of compressed air reduces the combined delay from turbine spool-up as well as engine cranking, thereby enabling a smooth and expedited engine restart to be provided in a hybrid vehicle. By expediting the delivery of maximal engine torque, the interim dependence on motor torque can be reduced, improving battery life. In addition, NVH issues and issues related to the loss of refinement in the vehicle wheel can also be reduced. Overall, hybrid vehicle performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.